The present invention relates to photoemissive devices, and more particularly to a solar energy converter comprising a plurality of photoemitting electrodes arranged to produce electrical power in usable form from solar energy.
In seeking a solution to the continuing problem of developing an efficient and practical source of electrical power which does not require the use of fossil fuels, a great deal of attention has been directed to the harnessing of solar energy. The art of converting such energy directly into electrical power has developed rapidly in recent years, particularly for applications in outer space where conventional power supplies are less than satisfactory. The need for an efficient, reliable, compact source of usable quantities of power at a reasonable cost has led to a proliferation of photoelectric devices utilizing various structures, materials, and techniques, principally relying on the photovoltaic properties of various semiconductors.
Some attempts have been made to utilize the photoemissive characteristics of metals and metal alloys to produce electrical currents of usable magnitude, but principally such photoemitters have been used in the detection or measurement of light, rather than in the generation of power. Typical of such attempts are U.S. Pat. Nos. 3,263,101 and 3,510,714 to C. W. Geer, both of which disclose photoemissive devices to convert solar energy into electrical current. These patents disclose the provision of large-area cathodes and anodes, wherein one or the other electrode is also a reflector which concentrates light. But such devices still have the disadvantage of being bulky, hard to handle, difficult to manufacture, and expensive. They are not, therefore, suitable either for use in outer space, where their bulk and manner of construction virtually precludes such an application, or for terrestrial use, where the large size required of such devices for useful levels of power, together with their expense, make them impractical and unsuitable.
As is well known, the phenomenon of photoemission, wherein electrons are ejected from a surface located in a vacuum and exposed to light energy, is exhibited by many materials, although the efficiency of this conversion varies with the material and with the wavelength of the light. Since light or shorter wavelengths contains a higher level of energy, more materials will exhibit photoemission in applications where ultraviolet light is present, as in outer space where the ultraviolet rays are not screened by the earth's atmosphere. However, some metals, such as cesium, as well as metal alloys and numerous semiconductive materials photoemit electrons upon exposure to light radiation in the visible wavelength range, with cesium cells being responsive, for example, to infra-red radiation as well.
To efficiently convert solar energy to electrical energy by photoemission, then, it is necessary to select a material which will serve as a cathode to emit electrons in the presence of light energy and to provide an anode for receiving the emitted electrons. The cathode must be highly illuminated by light energy of the appropriate wavelengths to insure emission of a large quantity of electrons, and the anode must be so located as to intercept a high proportion of the electrons. However, the geometrical arrangement of the anode and cathode must be such that the anode will not cast a shadow on the cathode so that maximum use of the cathode surface is obtained. Further, the anode itself must be so positioned that light does not fall on it, lest photoemission from the anode surface reduce the effective forward current flow alternately, the anode must be of a non-emitting material.
Numerous solar energy converters have been developed to take advantage of the ability of materials to photoemit electrons, but all have suffered disadvantages. Many are complex and time consuming to manufacture, many require exotic materials which are expensive and are difficult to handle, and many are limited in their applications and cannot be used, for example, in both outer space and terrestrial environments. In addition, many of the prior art devices encounter space charge limitations which prevent a significant flow of current, thus severely restricting their usefulness as power sources.